Example:
Using Google, go to Images tab. Right click on an image and select "Save image as". If you click on the file name, as you want to change the name, FF freezes.

Not only does it freeze but right clicking on the icon in the Dock only shows "Quit" (not Force quit) and that does nothing. Furthermore, it locks you out from being able to use the Apple symbol (top left) to get to "Force quit" that way. Only solution I have found so far is a forced reboot (i.e. power off/on).

I can reproduce this every time.

If anyone wants more info just let me know what you want to know. I suspect that, given Mojave has only been live a few hours, that a lot more people might run into this in the coming days.